Raman and infrared spectra of the incommensurate crystalNa2CO3

The complete Raman spectra of ${\mathrm{Na}}_2$ ${\mathrm{CO}}_3$ in its monoclinic (β), incommensurate (γ), and lock-in (δ) phases, as well as the far-infrared spectra in the γ and δ phases, have been measured and interpreted using symmetry and phenomenological arguments. Additional modes have been observed in the incommensurate phase as predicted by the superspace group selection rules of γ-${\mathrm{Na}}_2$ ${\mathrm{CO}}_3$. Two soft modes have been observed; one of them belongs to the monoclinic-to-incommensurate phase transition. Clear evidence has been found for the existence of a low-temperature commensurate (δ) phase.